The correct answer is: C. Cohesion
Cohesion is the force that holds together the particles of a substance. It is a type of intermolecular force that is responsible for the attraction between molecules of the same substance. Cohesion is what allows water to form droplets and why it is difficult to break a piece of metal.
In soldering, two metals are joined together by melting a filler metal and then flowing it into the joint between the two metals. The filler metal has a lower melting point than the two metals being joined, so it melts and flows easily. Once the filler metal has cooled, it forms a strong bond between the two metals.
The cohesion of the filler metal is what allows it to flow into the joint and form a strong bond. The cohesion of the two metals being joined also helps to create a strong bond. The two metals are attracted to each other, and this attraction helps to hold them together.
Viscosity is a measure of the resistance of a fluid to flow. It is a property of liquids and gases. Osmosis is the movement of water molecules through a semipermeable membrane. Surface tension is the tendency of a liquid surface to contract into the smallest possible area.
Viscosity, osmosis, and surface tension are not the properties that allow soldering of two metals. Cohesion is the property that allows soldering of two metals.